1. Field of the Invention
The present invention relates to a variable gain semiconductor circuit that is a variable gain circuit employed in a variable gain amplifier such as an automatic gain control (AGC) circuit or the like and realized with a semiconductor circuit. More particularly, this invention is concerned with a variable gain semiconductor attenuator whose distortion characteristic deteriorates very little, whose insertion loss is limited, and whose maximum attenuation is large.
In a receiving apparatus for radio communications or the like, a radio-frequency received signal is amplified by an amplifier, and then mixed with an oscillating signal, of a certain frequency, output from a local oscillator in order to produce an intermediate-frequency signal. For mixture, the received signal is required to have a constant amplitude. The amplitude of the received signal varies markedly depending on a receiving situation. The amplification factor of the amplifier is controlled by feeding back an output signal of a mixer so that a signal with a constant amplitude can always be output from the amplifier. This function is referred to as automatic gain control (AGC). For realizing the AGC function, the amplifier must have a variable gain capability for varying a gain. The present invention relates to a circuit for realizing the variable gain capability for varying the gain of a radio-frequency amplifier.
An example to be described below is a circuit employed in a receiving apparatus for radio communications or the like. In the example, an MES field-effect transistor (MESFET) is used as a semiconductor device. However, the present invention is not limited to this type of transistor. A MOSFET or the like can also be used.
2. Description of the Related Art
The performance of a circuit with a variable gain capability is required to provide a large variable gain range, a small insertion loss, a good distortion characteristic, and easiness in using the capability of the circuit. Amplification circuits whose amplification factors are variable all operate as circuits capable of varying their gain. Various circuits are used as circuits capable of varying gain. In general, a circuit referred to as an attenuator type is widely adopted. In the attenuator type circuit, an FET is connected between an input signal line and ground via a capacitive element and resistor. A control signal is applied to the gate of the FET. However, if this kind of attenuator type circuit is designed so that the quantity of attenuation dependent greatly on the properties of the transistor employed becomes large, the insertion loss of the circuit increases. In other words, the variable gain range and insertion loss have a trade-off relationship. This poses a problem that there is difficulty in greatly increasing the quantity of attenuation.
Another circuit capable of varying a gain includes a type of variable gain circuit that changes a quantity of attenuation, that is, a gain by varying the drain voltage of an FET operating as an amplifier, or a type of variable gain circuit that changes a gain by varying the gate voltage of an FET operating as an amplifier. In the type of variable gain circuit for varying the drain voltage, a control signal used to control the on or off state of the amplifier is usually output from a CMOS LSI needing a low current consumption. However, since an output signal of the CMOS LSI has poor driving ability, the control signal cannot be input directly to the drain of the FET. This type of variable gain circuit must therefore adopt such a method that a switch having a large current capacity is inserted between a power supply and the drain of the FET, and controlled using the control signal in order to change the quantity of attenuation. This poses a problem that the circuit becomes complex. In the type of variable gain circuit for varying the gate voltage of the FET, for increasing the quantity of attenuation, it is necessary to set a gate bias of the FET so that the operating point of the FET becomes as close to a pinch-off point thereof as possible. This poses a problem that the distortion characteristic of the circuit deteriorates. An actual circuit is realized by connecting pluralities of these types of circuits in series or connecting another circuits in series, but it still has the above problems. Even a combination of these types of circuits suffers from the same problems.